1. Field of the Invention
The present invention relates to a response probability calculating device and method, and a base station control device and communication system using the same, and more particularly to a response probability calculating method for use in a communication system for sending the same response probability to a plurality of communication terminals which report the terminal information at the response probability.
2. Related Art
For a Broadcast function and a Multicast function in a W-CDMA system, an MBMS (Multimedia Broadcast Multicast Service) is specified at Release 6 in a 3 GPP (3rd Generation Partnership Project).
In the MBMS, the same data is sent to all the cells simultaneously. The modes of data transmission include a PTM (Point-to-Multipoint) and a PTP (Point-to-Point). The PTM is a transmission mode of sending the data to every communication terminal (hereinafter simply referred to as a terminal) within the cell through one RL (Radio Link), in which a common channel SCCPCH (Secondary Common Control Physical Channel) set in each cell is used as a physical channel. Since this PTM can send the data through one RL, the radio resources can be saved. However, to cover the entire cell, a fully high sending power is required. Accordingly, for the terminals other than near the cell end (cell boundary), the data transmission is performed at excessively high power level.
On the other hand, the PTP is a transmission mode of transmitting the data to one terminal through one RL, in which a high speed physical downlink shared channel (HS-PDSCH) or a dedicated physical channel DPCH (Downlink Physical Channel) is used as the physical channel. The HS-PDSCH is a data channel of an HSDPA (High Speed Downlink Packet Access) method, and an AMCS (Adaptive Modulate and Coding Scheme) of changing a transmission rate according to a received signal quality is applied, whereby the data can be transmitted at higher speed than the DPCH.
A well-known inner loop control (Inner Loop Power Control) of changing a transmission power according to the received signal quality is applied to the DPCH. Accordingly, the PTP can save the radio resource per RL according to the received signal quality. However, since the total amount of sent data increases according to the number of MBMS receiving users, the transmission power is greater when there are a large number of MBMS data receiving users.
The RNC (Radio Network Controller or base station control device) measures the number of MBMS receiving users (N_count) for each cell in performing the MBMS. This process is called counting. In this counting, the response probability AP (Access Probability) of equal value is sent to all the users. Each user desiring to receive the MBMS data responds according to the response probability AP. For example, when AP is 10%, each MBMS receiving user responds at a probability of 10%.
The RNC measures N_count, using the number of users responding simultaneously at a certain AP (hereinafter a response number), and can decide the presence or absence of sending the MBMS data, the selection between the PTM transmission and the PTP transmission, and the switching between the PTM transmission channel and the PTP transmission channel, based on the measured N_count.
An example of measuring N_count with an arithmetical mean of two response numbers is described below. Assuming that AP is 10% and the simultaneous response number is 9 at the first time, and AP is 20% and the response number is 22 at the second time, N_count is measured as 100.N_count=(1/2)*{(9/10%)+(22/20%)}=100Where * denotes the multiplication (the same sign is used in the following).
The RNC wishes to measure the number of MBMS receiving users efficiently at the time of starting the MBMS service. Also, to avoid congestion or collision of response signals in the link due to responses, the response number is wished to be made a certain upper limit value or less. Also, to reduce a measurement error, the response number is wished to be secured to a lower limit value or more (e.g., one or more).
To avoid occurrence of the congestion or the collision in the link because the response number exceeds the upper limit value, the small AP is firstly sent, and if the response number is less or zero, the AP is increased. A method for increasing the AP may involve increasing the AP according to a linear function as shown in the following formula (1), for example. Herein, n denotes the number of updating the AP, a denotes the initial value, and S denotes the increase step width.AP(n)=a+S*(n−1)  (1)
Besides, another method for increasing the AP according to an exponential function as shown in the following formula (2) is considered. Where B is a base of exponent, and the other parameters are defined in the same manner as the formula (1).AP(n)=a*B^(n−1)  (2)
As the related arts, there are Japanese Patent Application Laid-Open No. 2005-252506, National Publication of International Patent Application No. 2006-505979, National Publication of International Patent Application No. 2006-515496, National Publication of International Patent Application No. 2006-515737, and National Publication of International Patent Application No. 2006-526316.
If the AP is increased every time of sending, the response number exceeds the lower limit value at once, whereby the number of MBMS receiving users can be measured by a small number of sending the AP. However, especially when the number of MBMS receiving users is large, there is a problem that the response number is likely to exceed the upper limit value.
On the other hand, if the AP is increased bit by bit, the response number can be suppressed to the upper limit value or less. However, especially when the number of MBMS receiving users is small, the response number is small, whereby there is a problem that it takes a long time to make the measurement.
Accordingly, the conventional AP calculation method has the following problem. With the method for increasing the AP according to the linear function, if the increase step width is large, the response number is likely to exceed the upper limit value at once. On the other hand, if the increase step width is small, it takes a very long time to make the measurement. With the method for increasing the AP according to the exponential function, the measurement in short time can be expected, but since the AP rapidly increases according to the number of updating the AP, the response number rapidly increases halfway, and easily exceeds the upper limit value.
As described above, the response number is suppressed to a permissible value or less by updating appropriately the AP, whereby it is a common problem to avoid occurrence of the congestion or the collision due to responses, and acquire a predetermined number of responses or more in a short time in the communication system, wired or wireless, in which the AP is sent to a plurality of terminals and each terminal reports the terminal information according to the AP.
For example, in a radio communication system, the received signal quality of the radio channel is reported according to the AP, and the transmission power is controlled using this received signal quality. Also, in a wired communication system, the status information of the terminal may be collected.